A brake fluid pressure control apparatus for executing an ABS control by controlling a brake fluid pressure (hereinafter referred to as a wheel cylinder pressure) within a wheel cylinder has been mounted to a wide variety of vehicles. Such brake fluid pressure control apparatus generally includes a normally opened solenoid valve (pressure increasing valve) and a normally closed solenoid valve (pressure reducing valve), the normally opened solenoid valve being provided at a hydraulic pressure circuit connecting the master cylinder to the wheel cylinder and the normally closed solenoid valve being provided at a hydraulic pressure circuit connecting the wheel cylinder to the reservoir. In this configuration, a pressure reducing control, a sustaining control and a pressure increasing control for the wheel cylinder pressure are executed by controlling the pressure increasing valve and the pressure reducing valve respectively.
The ABS control is generally started and executed for the vehicle wheel at which a predetermined ABS control start condition is established. Specifically, the ABS control is achieved by executing the pressure increasing control at least after the pressure reducing control is executed. Then, on the pressure increasing control during the ABS control, when the ABS control start condition is established again, the executing pressure increasing control is terminated, at the same time a next ABS control (a pressure reducing control of a next ABS control) is continuously started.
Generally, plural ABS controls are executed in succession over plural control cycles, one of which is a time period between a point at which the ABS control start condition is established and a point at which the next ABS control start condition is established.
Recently, a smooth (linear) control increasing the wheel cylinder pressure has been in demand. Hereinafter, such control will be referred to as a linear pressure increasing control in this specification. In order to cope with this situation, as disclosed in JP2003-19952A, a linear solenoid valve, especially a normally opened linear solenoid valve, has been adopted as a pressure increasing valve for the brake fluid pressure control apparatus. Specifically, such linear solenoid valve can linearly control a value, which is obtained by subtracting a wheel cylinder pressure from the master cylinder pressure, by linearly controlling a conducting electric current value.
When the normally opened linear solenoid valve is used, a pressure difference corresponding to a suction force varies in proportion to the conducting electric current value (command current). Hereinafter such pressure difference will be referred to as a command pressure difference in this specification. Accordingly, the normally opened linear solenoid valve serving as the pressure increasing valve is controlled so as to: disconnect the hydraulic pressure circuit between the master cylinder and the wheel cylinder, when the command pressure difference determined in accordance with the conducting electric current value is larger than an actual pressure difference; and connect the hydraulic pressure circuit between the master cylinder and the wheel cylinder, when the command pressure difference is smaller than the actual pressure difference.
In other words, in a condition where the pressure reducing valve is maintained in a closed state, when the command pressure difference is larger than the actual pressure difference, the wheel cylinder pressure is sustained. Further, when the command pressure difference is smaller than the actual pressure difference, because the brake fluid flows from the master cylinder into the wheel cylinder, the wheel cylinder pressure increases, at the same time, the actual pressure difference is reduced, and when the actual pressure difference becomes equal to the command pressure difference, the actual pressure difference and the command pressure difference are balanced.
Specifically, in order to obtain a smooth increase of the wheel cylinder pressure immediately after a starting point of the linear pressure increasing control by use of the normally opened linear solenoid value serving as the pressure increasing valve, while the pressure reducing valve is maintained in a closed state, at the starting point of the linear pressure increasing control, the conducting electric current value of the normally opened linear solenoid valve (pressure increasing valve) is set to an electric current value corresponding to the actual pressure difference, and subsequently the conducting electric current value needs to be reduced in a constant slope. The electric current value is a conducting electric current value for controlling the command pressure difference so as to agree with the actual pressure difference (hereinafter referred to as an actual pressure difference equivalent to a current value). Thus, from the starting point of the linear pressure increasing control, the actual pressure difference is smoothly reduced, as a result, during the linear pressure increasing control, the wheel cylinder pressure can be smoothly increased.
On the other hand, when a value of the conducting electric current value, which is reduced during the linear pressure increasing control at the starting point of the linear pressure increasing control, is set to be larger than the actual pressure difference equivalent to a current value, during a period between the starting point of the linear pressure increasing control and a point at which the command pressure difference, which has been reduced, becomes equal to the actual pressure difference, the normally opened linear solenoid value is maintained in a closed state in order to sustain the wheel cylinder pressure. Specifically, a start of increase of the wheel cylinder pressure may delay. This delay will be referred to as an increase start delay of the wheel cylinder pressure.
Further, when a value of the conducting electric current value, which is reduced during the linear pressure increasing control, is set at the starting point of the linear pressure increasing control to a value that is smaller than the actual pressure difference equivalent to a current value, during a period in which the value of the actual pressure difference is reduced due to the brake fluid flowing from the master cylinder to the wheel cylinder so as to be equal to the command pressure difference, the normally opened linear solenoid value is maintained in an opened state, so that the wheel cylinder pressure rapidly increases. This rapid increase will be referred to as a rapid increase of the wheel cylinder pressure.
Thus, in order to smoothly increase the wheel cylinder pressure immediately after the starting point of the linear pressure increasing control, an actual pressure difference equivalent to a current value, at the starting point of the linear pressure increasing control (the actual pressure difference at the starting point of the linear pressure increasing control), needs to be accurately calculated. This actual pressure difference can be detected by means of a sensor for detecting the master cylinder pressure, or by means of both of a sensor for detecting the wheel cylinder pressure and the sensor for detecting the master cylinder pressure. However, because such configuration using the two sensors may increase the production cost of the apparatus and make it difficult to secure reliability of the sensors, the configuration using two sensors may not be generally acceptable.
Accordingly, there was a need to accurately estimate an actual pressure difference during the ABS control without using the abovementioned sensor, and various types of methods for accurately estimating the actual pressure difference during the ABS control without using the abovementioned sensor have been proposed so far.
When the vehicle is driven on a road (hereinafter referred to as a split road) having two types of road surfaces, one road surface including a high friction property (hereinafter referred to as a high μ road surface) and the other surface including a low friction property (hereinafter referred to as a low μ road surface), and one of right and left wheels contacts the high μ road surface, and the other of the right and left wheels contacts the low μ road surface, and in this situation, suppose the driver makes a relatively strong braking operation; the ABS control is started and executed on the front wheels contacting the low μ road surface (hereinbelow referred to as a low μ side front wheel), and then the ABS control is also started and executed on the front wheel contacting the high μ road surface (hereinbelow referred to as a high μ side front wheel).
In this situation, a control is executed for restraining the increase of the wheel cylinder pressure at the high μ side front wheel, at which the ABS controls has not been started, during a time period between a point in time at which the ABS control is started at the low μ side front wheel and another point in time at which the ABS control is started at the high μ side front wheel, in other words in a time period during which the ABS control has been executed only at the low μ side front wheel, and this control has been widely known and will be referred to as a yaw moment control in the specification. Because of this yaw moment control, a yawing moment generated due to a difference between a braking force (a friction force between a tire and a road surface) generated at the right front wheel and a braking force generated at the left front wheel is reduced, the vehicle can be prevented from being driven under an unstable condition.
Specifically, in a case where the yaw moment control is executed, as the brake fluid pressure control for the high μ side front wheel, the yaw moment control will be executed first, and then the ABS control will be executed instead of the yaw moment control.
In this embodiment, a method is provided for accurately estimating an actual pressure difference at the high μ side front wheel during the ABS control is executed for the high μ side front wheel, when the brake fluid pressure control for the high μ side front wheel is changed from the yaw moment control to the ABS control.
A need thus exists to provide a brake fluid pressure control apparatus for appropriately estimating a pressure difference between the master cylinder pressure and the wheel cylinder pressure at the second front wheel, which is under the ABS control, in a case where the brake fluid pressure control for the second front wheel is changed from the yaw moment control to the ABS control while the first front wheel is under the ABS control.